JOURNAL OF FERMENTATION AND BIOENGINEERING Vol. 68, No. l, 71-73. 1989

Sparger Position Effect over kLa in Bench and Pilot Stirred-Tank Fermentors

ALFREDO MARTINEZ, ENRIQUE GALINDO, AND MIGUEL SALVADOR*

Departamento de Bioingenier[a, Centro de Investigaciones sobre lngenier[a Gendtica y Biotecnolog[a, Universidad Nacional Aut6noma de Mdxico, A.P. 510--3, Cuernavaca, Mor. 62271, M~xico

Received 16 August 1988/Accepted 4 May 1989

The present investigation describes the influence of sparger position in relation to the volumetric oxygen transfer coefficient of the liquid film (kLa) in tanks of four different sizes. It was found that the influence of the sparger position on kLa is more evident as the scale increases.

The information on gas-liquid contact in stirred tank fermentors is very extensive; however, work dealing with the effect of the position of the gas sparger on the volumetric oxygen transfer coefficient of the liquid film (k~a) is very scarce. Flynn and Lilly (1) described a method to control the dissolved oxygen tension in microbial cultures by means of an oxygen controller acting on a moving gas sparger. Nienow et al. (2) have recommended placing the sparger 0.04 tank diameter times below the bottom of the impeller blades in order to obtain better gas dispersion. Mehta and Sharma (3), although they did not indicate the position of the gas sparger, found that when the ratio be- tween the distance of the impeller to tank bottom, C, to the tank diameter, T, was varied from 0.25 to 0.34, the k~a value increased, whereas when varied from 0.34 to 0.5 the k~a value remained constant.

Due to the virtual lack of data, it was decided to study the effect of the gas sparger position more extensively. Ad- ditionally, the study was carried out at four different scales of mechanically agitated fermentors, in order to evaluate the effect of scale.

Fermentors with tank diameters of 0.11, 0.212, 0.416 and 0.69m were used. The distance between the gas sparger and the impeller was varied. For comparative pur- poses the distance between the sparger and the impeller disc, z, to the impeller diameter, D, was chosen to cor- relate the k~a values.

All experiments were carried out with Rushton turbine impellers with six fiat blades and tap water at 29°C. Air or nitrogen was introduced through a ring sparger having holes discharging parallel to the shaft. The geometric dimensions of the employed fermentors are given in Table 1, as are the operating conditions.

k~a measurements were performed according to the dynamic gassing-in liquid method (Sobotka, M. et al., An- nu. Rep. Ferment. Proc., Academic Press, London, vol. 5, p. 127, 1982), using nitrogen as the displacement inert gas. The oxygen tension was measured by a galvanic probe (New Brunswick Scientific, 900 series) and a dissolved oxygen analyzer (New Brunswick Scientific, model DO-40). Figure 1 illustrates schematically a typical tank and the an- ciliary equipment employed.

In order to evaluate if the oxygen probe position had any effect on the k~a values, some determinations were car-

* Corresponding author.

71

ried out in the 0.416m diameter tank at different z /D ratios. The probe height was first kept constant, and then later varied at the same time as the z /D ratio. Similar kLa values were found in both cases (data not shown). Mixing times in similar experimental conditions have been reported (4) as very small in comparison with the time re- quired for kLa measurement. Therefore, it could be assumed that the oxygen tension was uniform throughout the vessel, and that the probe position did not affect the kLa measurements in a significant way.

Figure 2 shows the k~a values obtained at different z /D ratios for the vessels of 0.11, 0.212 and 0.69 m in diameter. Figure 3 shows the k~a values for the vessel with a tank diameter of 0.416 m. The differences in the magnitude of the kLa values were in general agreement with the differences in the calculated volumetric power input to the gassed systems. The orders of magnitude of the k~a values were similar to those obtained by other workers (5, 6) where experimentation was carried out under similar condi- tions.

TABLE 1. Geometric dimensions of the equipment employed and working conditions

Symbols used in o o /x • • figures

T 0.110 0.212 0.416 0.416 0.690 H / T 1.435 1.336 1.769 1.769 1.357 V 0.00015 0.010 0.I00 0. I00 0.350 D/T 0.44 0.33 0.33 0.47 0.40 WilD 0.19 0.21 0.19 0.20 0.20 L i / D 0.25 0.25 0.28 0.25 0.25 Nb 4 4 4 4 4 W b / T 0.11 0.11 0.09 0.09 0.10 Ds/D 0.75 0.75 0.75 0.52 0.80 nh 16 36 36 36 36 dh 0.79 0.79 3.17 3.17 4.76 dp 3.18 4.76 19.05 19.05 25.4 C s / T 0.25 0.09 0.13 0.13 0.14 z / D 0.15-1.00 0.15-1.00 0.15-0.75 0.15-0.50 0.15-0.60 C / T 0.32-0.69 0.15-0.44 0.18-0.38 0.20-0.37 0.20-0.38 C p / T a 0.58 0.36 0.38 0.48 0.44 N 13.33 6.67 4.17 4.17 4.00 vvm 1.0 1.0 1.0 1.0 1.0

a The probe height corresponds to that when the impeller is z/D=0.75.

72 MARTINEZ ET AL. J. FERMENT. BIOENG.,

T g}

N o

O .J

Wb

®

T

m

[®

A r

FIG. 1. Schematic diagram of the equipment used. O Tank; (~) Gas sparger; (~) Oxygen probe; (~) Oxygen analyzer; (~) Impeller (variable height); (~) Valve; (Z) Rotameter.

For the tanks with diameters of 0.11 and 0.212m, the k~a was constant over a wide range of z /D ratios. This fact can probably be explained in terms of the nearly perfect mixing which prevailed in operating conditions as reported by Joshi et al. (4). Only when the impeller approached the liquid surface did the mass transfer decrease, probably

I

NO K

T = 0 . 1 1 m

O 0 0

7 " I O 0 O

(88.4)

~ m

( 7 . 6 7 )

n

T =0.218rn o i - -

0 0.2 0.4 o.s 0.8 ~ .o

z / D

FIG. 2. kLa values at different z/D ratios for tank diameters of 0.11, 0.212 and 0.69 m. Numbers in parentheses indicate the (FIo)FL/ Fl G ratio (Symbols as in Table 1).

4 . 0

3 .0

2 . 0

( 13.02 )

T = 0.416 m

O ~

0 0A 0.2 0 .3 0 .4 0,5 0 .6 0,7 0.8

z / D

FIG. 3. kLa values at different z/D ratios for the 0.416m diameter tank. Numbers in parentheses indicate the (FIo)FL/FIo ratio (Symbols as in Table 1).

because the mixing was then defective and the system began to perform more like a bubble column. In the tank with a diameter of 0.69 m, the kLa declined as the z /D ratio increased. In accordance with Nienow et al. (2), under our experimental conditions the impeller was under the flow regime of loading, i.e., the ratio of the gas flow number to reach the flow regime of loading, (FIG)FL, to the operation gas flow number, Flo, exceeded unity. In this situation the flow patterns were dominated by the agitation.

In the case of the experiments with a 0.416 m diameter tank (D/T=0.33), the kLa declined as the z /D ratio increased. In those experiments the flow regimes showed flooding conditions and consequently the flow patterns were dominated by aeration according to Nienow et al. (2). In the case of D/T=0.47 , the kLa declines drastically as the z /D ratio increases. Nevertheless, in this latter case the flow regimes were of loading (2) and the flow patterns were dominated by agitation.

The decrease in the kLa value was more drastic as z in- creased and as the scale increased, and also more drastic when the flow patterns were dominated by the agitation. These facts suggest that the decay in the kLa value when the z /D ratio increases was very probably due to a higher bub- ble coalescense as z increases. The trend found in this study for the 0.21 m diameter tank agrees with that reported by Mehta and Sharma (3) for a tank with a 0.20 m diameter. In a larger tank (0.40 m diameter), the authors (3) observed an increase in the kLa value as the C/Tratio in- creased. Unfortunately, they did not indicate the position of the gas sparger and the experimental conditions used in

VoL 68, 1989 NOTES 73

this w o r k were different. This makes it imposs ib le to strictly c o m p a r e the da t a wi th our da t a for tanks wi th 0.416 and 0.69 m in d iamete r which clear ly showed a decrease in kLa as z / D increased.

It can be conc luded that even t h o u g h agi ta t ion d o m - inates f low pat te rns , the sparger pos i t ion plays an im- po r t an t role in the mass t ransfer p h e n o m e n o n . A l t h o u g h it can no t be ru led ou t tha t the changes we observed cou ld be associa ted wi th changes in the p o w e r d raw to the system, the da t a emphas ize the i m p o r t a n c e o f the pos i t ion o f the gas sparger in s t i r red- tank f e rmen to r s scale-up. In o rder to max imize mass t ransfer , it is suggested tha t the gas sparger be pos i t ioned as close as possible to the b o t t o m o f the im- peller blades. This tends to avo id bubb le coalescence and induces the bubbles f r o m enter ing direct ly in the impel ler blades.

Final ly , it shou ld be po in t ed ou t tha t the sparger posi- t ion (usually no t specified in l i tera ture data) may be ano the r reason for the d i sagreement in kLa values repor ted under s imilar ope ra t ing condi t ions .

NOMENCLATURE

C : distance between impeller and tank bottom, m Cp : distance between oxygen probe and tank bottom, m Cs : distance between gas sparger and tank bottom, m D : impeller diameter, m Ds : gas sparger ring diameter, m dh : hole diameter, mm dp : inner diameter of ring sparger, mm Flo : gas flow number, (Qc, N ID 3), dimensionless H : liquid height, m kLa : volumetric oxygen transfer coefficient in liquid film, s -~ Li : impeller blade length, m N : agitator speed, s Nb : number of baffles

nh : number of holes in gas sparger V : working volume, m a vvm : gas volume/rain, x liquid volume without aereation, m 3 air m -3

liquid min- T : tank diameter, m Wb : baffle width, m Wi : impeller blade width, m z : distance between sparger and disc impeller, m Subscripts FL : flooded

The authors acknowledge, with thanks, Mr. M.A. Caro for his valuable assistance in the work with the pilot plant fermentors and Dr. M. Griot for the helpful comments and revision of the manuscript.

REFERENCES

1. Flynn, D. S. and Lilly, M. D.: A method for the control of the dissolved oxygen tension in microbial cultures. Biotechnol. Bioeng., 9, 515-531 (1967).

2. Nienow, A.W., Haozhung, W., Houxing, L., and Allsford, K. V.: The advantage of using large ring spargers with standard Rushton turbines in gassed reactors. World Congress III of Chemical Engineering, Tokyo, Japan, Vol. III, p. 354-357 (1986).

3. Mehta, V. D. and Sharma, M. M.: Mass transfer in mechanically agitated gas-liquid contactors. Chem. Eng. Sci., 26, 461-479 (1971).

4. Joshi, J.B., Pandit, A.B., and Sharma, M.M.: Mechanically agitated gas-liquid reactors. Chem. Eng. Sci., 37, 6, 813-844 (1982).

5. Yoo, Y. J. and Hong, J.: Bubble entrainment aeration for high- foaming fermentation. Biotechnol Bioeng., 28, 756-760 (1986).

6. Nishikawa, M., Nakamura, M., Yagi, H., and Hashimoto, K.: Gas absorption in aerated mixing vessels. J. Chem. Eng. Jpn. 14, 3, 219-229 (1981).